In a wireless communications system, due to impact of factors such as a transmission distance and a Doppler frequency shift, strength of a signal received by a receiver varies in a large dynamic range. A strong signal tends to saturate the receiver. A weak signal cannot be detected by a demodulator and therefore, tends to be lost. Currently, signals having a wide disparity in strength may be adjusted into a proper range by using an automatic gain control (AGC for short) method, so that a signal to noise ratio (SNR for short) of a signal that is output to a baseband is optimal.
For example, when a receiver in a wireless local area network (WLAN for short) receives a signal, a power gain is first adjusted for the received signal, so that the signal enters into an analog to digital converter at a proper power. Then an analog signal is converted into a digital signal, so that digital processing is further performed on the received signal. In an Institute for Electrical and Electronics Engineers (IEEE) 802.11n standard, AGC estimation is performed on the received signal by using a legacy short training field (L-STF for short) and a high throughput short training field (HT-STF for short) in a preamble. In an IEEE 802.11ac standard, AGC estimation is performed on the received signal by using an L-STF and a very high throughput short training field (VHT-STF for short) in a preamble.
However, when AGC estimation is performed on a received signal by using an L-STF and an HT-STF, or an L-STF and a VHT-STF according to the foregoing method, system overheads are relatively large. The system overheads may be further reduced when a good AGC effect is ensured.